Originally Posted by ted_major

two water bottles, one insulated, one uninsulated
Pictured above are two water bottles, insulated on the right, uninsulated on the left. Both bottles are labelled 750 mL (no, I didn't measure the volume; I'll assume they're close enough to labelled volume to not matter). The insulated bottle is approx. 5/8" taller than the uninsulated one. So yes, there is a difference in the size of an insulated bottle vs uninsulated of the same volume (and by extension, bottles of the same external dimensions will have different volumes). For me, the difference isn't enough to matter (if you have a small main triangle or use a frame bag and bottles are a tight fit, that 5/8" might make a difference).

As far as the effectiveness, I find that when I start out with a bottle filled with ice and topped off with cold water on a west Alabama summer day (90-100F temp, high humidity), after about 3 hours the insulated bottle isn't cold anymore, but it is still noticeably cooler than ambient temps. I don't usually bother to freeze bottles the night before, so I can't comment on how much more effective that might be compared to just filling with ice.